One of the most important requirements of the next-generation wireless communication system is to support a requirement for a high data transfer rate. To this end, research is being carried out on various technologies, such as Multiple Input Multiple Output (MIMO) and Cooperative Multiple Point transmission (CoMP), and a relay, but the most basic and stable solution is to increase the bandwidth.
However, frequency resources are now in a saturation state, and various technologies are partially being used in wide-ranging frequency bands. For this reason, as a scheme for securing a wide bandwidth in order to satisfy requirements for a higher data transfer rate, a Carrier Aggregation (CA) having a concept in which each of scattered bands is designed to satisfy a basic requirement capable of operating an independent system and a number of bands are bundled into one system is being introduced. Here, a band that may be independently operated is defined as a Component Carrier (CC).
In order to support an increasing transmit capacity, the bandwidth of the recent 3GPP LTE-A or 802.16m is extending up to 20 MHz or higher. In this case, one or more CCs are aggregated to support a wide band. For example, if one CC corresponding to a bandwidth of 5 MHz, four carriers are aggregated in order to support a maximum of a 20 MHz bandwidth.
As described above, the carrier aggregation system uses a plurality of CCs. Here, a CC on which downlink control information is received from a base station and a CC on which downlink data is received from the base station may be different configured in user equipment. Alternatively, a link between a CC on which downlink control information is received and a CC on which an uplink signal is transmitted may comply with different linkage from a link defined in the existing LTE. This scheduling method is called cross-carrier scheduling. In the case where the cross-carrier scheduling is applied, if it is not identified that downlink control information transmitted by a base station is about what CC, it is difficult for user equipment to communicate with the base station. There is a need for a method of user equipment efficiently communicating with a base station by providing a method of indexing each CC in a carrier aggregation system.